The child with Acute Lymphocytic Leukemia (ALL) is at risk for the development of neuropsychological dysfunction. The Central Nervous System (CNS) aspects of the disease, some aspects of treatment and the vulnerability of the ALL child to CNS infection all contribute to his high risk status. The proposed study will investigate the neuropsychological status of children treated for ALL in a longitudinal fashion by means of serial neuropsychological, audiological and electroencephalographic examinations and by comparison with control groups consisting of siblings of leukemic children and comparably aged children with Wilm's Tumor. All children admitted to St. Jude Children's Research Hospital for treatment of ALL between the ages of four and twelve (40-50 per year) will be serially evaluated for five years by means of the Reitan-Indiana Neuropsychological Test Battery for Children or the Halstead Neuropsychological Test Battery for Children as well as electroencephalograms and the Staggered Spondeic Word Test. Identical procedures will be administered to the control groups. If alterations in neuropsychological functioning do occur, they are expected to be variable across age groups, consistent within age groups. A thorough statistical analysis of the data will result in the detection of areas of neuropsychological dysfunction and determination of the etiology of any dysfunctions which occur, i.e., whether they result from the disease and its treatment, complications, or other factors. The determination of specific areas of neuropsychological dysfunction will facilitate the development of remediation strategies applicable to the classroom. Academic achievement will be monitored yearly to assess the effectiveness of any remediation initiated. The continued success of the treatment for ALL suggests that the neuropsychological ramifications of the disease, its treatment and complications will be important to an expanding population of children in the future. In addition, by thoroughly examining a population at risk for CNS dysfunction during the high risk period, new knowledge can be obtained not only concerning the population of ALL children but also of the developing CNS in general.